While the Mixed Armor rules apply only to Armor Saves (Presumably, you can apply invul saves or cover saves wherever you want), I feel I should point out how the rule works, just for the sake of clarity.
A lot of people assume Mixed Armour works the same as Mixed Toughness. This, fortunately, is not the way it works.
When you have a unit with Mixed Armour saves, wounds are applied to the majority save value *first* (though the owner still gets to pick who saves against what weapons), however you still never apply more than one wound to a single wound model unless there's an overwhelming amount of incoming wounds.
So if you have a unit of 10 men, 6 of which have Save A, and 4 of which have Save B.
The unit takes 5 wounds. These are all applied to Save A.
The unit takes 7 wounds. 6 of these are applied to Save A, 1 is applied to Save B
So minority save still get used, but only when there are more wounds than majority save models.
But this applies only to *armour* saves. Models with the same armour save but different invul saves have their saves distributed however the owner likes, so long as the rules about distributing wounds are followed.